Chapter 7 Hosting

Once you have developed your Plumber API, the next step is to find a way to host it. If you haven’t dealt with hosting an application on a server before, you may be tempted to run the run() command from an interactive session on your development machine (either your personal desktop or an RStudio Server instance) and direct traffic there. This is a dangerous idea for a number of reasons:

Your development machine likely has a dynamic IP address. This means that clients may be able to reach you at that address today, but it will likely break on you in the coming weeks/months.

Networks may leverage firewalls to block incoming traffic to certain networks and machines. Again, it may appear that everything is working for you locally, but other users elsewhere in the network or external clients may not be able to connect to your development machine.

If your Plumber process crashes (for instance, due to your server running out of memory), the method of running Plumber will not automatically restart the crashed service for you. This means that your API will be offline until you manually login and restart it. Likewise if your development machine gets rebooted, your API will not automatically be started when the machine comes back online.

This technique relies on having your clients specify a port number manually. Non-technical users may be tripped up by this; some of the other techniques do not require clients specifying the port for an API.

This approach will eternally run one R process for your API. Some of the other approaches will allow you to load-balance traffic between multiple R processes to handle more requests. RStudio Connect will even dynamically scale the number of running processes for you so that your API isn’t consuming more system resources than is necessary.

Plumber uses the interactive() function as a heuristic for whether it should behave in a more convenient mode or in a more robust, secure mode. This function is used as the default for some parameters on run() which could pose a security hazard if enabled on a public server.

Most importantly, serving public requests from your development environment can be a security hazard. Ideally, you should separate your development instances from the servers that are accessible by others.

For these reasons and more, you should consider setting up a separate server on which you can host your Plumber APIs. There are a variety of options that you can consider.

7.1 DigitalOcean

DigitalOcean is an easy-to-use Cloud Computing provider. They offer a simple way to spin up a Linux virtual machine and access it remotely. You can choose what size machine you want to run – with options ranging from small machines with 512MB of RAM for a few dollars a month up to large machines with dozens of GB of RAM – and only pay for it while it’s online.

Plumber includes helper functions that enable you to automatically provision a Plumber server and deploy your APIs to it. So in order to setup a Plumber server running on DigitalOcean, you’ll follow these steps:

Setup an SSH key and deploy the public portion to DigitalOcean so you’ll be able to login to your server.

Install the analogsea R package and run a test command like analogsea::droplets() to confirm that it’s able to connect to your DigitalOcean account.

Run mydrop <- plumber::do_provision(). This will start a virtual machine (or “droplet”, as DigitalOcean calls them) and install Plumber and all the necessary prerequisite software. Once the provisioning is complete, you should be able to access port 8000 on your server’s IP and see a response from Plumber.

Install any R packages on the server that your API requires using analogsea::install_r_package().

You can use plumber::do_deploy_api() to deploy or update your own custom APIs to a particular port on your server.

(Optional) Setup a domain name for your Plumber server so you can use www.myplumberserver.com instead of the server’s IP address.

(Optional) Configure SSL

Getting everything connected the first time can be a bit of work, but once you have analogsea connected to your DigitalOcean account, you’re now able to spin up new Plumber servers in DigitalOcean hosting your APIs with just a couple of R commands. You can even write scripts that provision an entire Plumber server with multiple APIs associated.

7.2 RStudio Connect

RStudio Connect is an enterprise publishing platform from RStudio. It supports push-button publishing from the RStudio IDE of a variety of R content types including Plumber APIs. Unlike all the other options listed here, RStudio Connect automatically manages the dependent packages and files your API has and recreates an environment closely mimicking your local development environment on the server.

RStudio Connect automatically manages the number of R processes necessary to handle the current load and balances incoming traffic across all available processes. It can also shut down idle processes when they’re not in use. This allows you to run the appropriate number of R processes to scale your capacity to accommodate the current load.

Conflict of interest: the primary author of plumber and this book works for RStudio on RStudio Connect.

7.3 Docker (Basic)

Docker is a platform built on top of Linux Containers that allow you to run processes in an isolated environment; that environment might have certain resources/software pre-configured or may emulate a particular Linux environment like Ubuntu 14.04 or CentOS 7.3.

We won’t delve into the details of Docker or how to setup or install everything on your system. Docker provides some great resources for those who are looking to get started. Here we’ll assume that you have Docker installed and you’re familiar with the basic commands required to spin up a container.

In this article, we’ll take advantage of the trestletech/plumber Docker image that bundles a recent version of R with the most recent version of plumber pre-installed (the underlying R image is courtesy of the rocker project). You can get this image with a

docker pull trestletech/plumber

Remember that this will get you the current snapshot of Plumber and will continue to use that image until you run pull again.

7.3.1 Default Dockerfile

We’ll start by just running a single Plumber application in Docker just to see things at work. By default, the trestletech/plumber image will take the first argument after the image name as the name of the file that you want to plumb() and serve on port 8000. So right away you can run one of the examples that’s included in plumber as it is already installed on the image.

-p 8000:8000 says to map port 8000 from the plumber container (which is where we’ll run the server) to port 8000 of your local machine

trestletech/plumber is the name of the image we want to run

/usr/local/lib/R/site-library/plumber/examples/03-mean-sum/plumber.R is the path inside of the Docker container to the Plumber file you want to host. You’ll note that you do not need plumber installed on your host machine for this to work, nor does the path /usr/local/... need to exist on your host machine. This references the path inside of the docker container where the R file you want to plumb() can be found. This mean-sum path is the default path that the image uses if you don’t specify one yourself.

This will ask Plumber to plumb and run the file you specified on port 8000 of that new container. Because you used the -p argument, port 8000 of your local machine will be forwarded into your container. You can test this by running this on the machine where Docker is running: curl localhost:8000/mean, or if you know the IP address of the machine where Docker is running, you could visit it in a web browser. The /mean path is one that’s defined in the plumber file we just specified – you should get an single number in an array back ([-0.1993]).

If that works, you can try using one of your own plumber files in this arrangement. Keep in mind that the file you want to run must be available inside of the container and you must specify the path to that file as it exists inside of the container. Keep it simple for now – use a plumber file that doesn’t require any additional R packages or depend on any other files outside of the plumber definition.

For instance if you have a plumber file saved in your current directory called api.R, you could use the following command

You’ll notice that we used the -v argument to specify a “volume” that should be mapped from our host machine into the Docker container. We defined that the location of that file should be at /plumber.R, so that’s the argument we give last to tell the container where to look for the plumber definition. You can use this same technique to share a whole directory instead of just passing in a single R file; this approach is useful if your Plumber API depends on other files.

You can also use the trestletech/plumber image just like you use any other. For example, if you want to start a container based on this image and poke around in a bash shell:

docker run -it --rm --entrypoint /bin/bash trestletech/plumber

This can be a handy way to debug problems. Prepare the command that you think should work then add --entrypoint /bin/bash before trestletech/plumber and explore a bit. Alternatively, you can try to run the R process and spawn the plumber application yourself and see where things go wrong (often a missing package or missing file).

7.3.2 Custom Dockerfiles

You can build upon the trestletech/plumber image and build your own Docker image by writing your own Dockerfile. Dockerfiles have a vast array of options and possible configurations, so see the official docs if you want to learn more about any of these options.

A couple of commands that are relevant here:

RUN runs a command and persists the side-effects in the Docker image you’re building. So if you want to build a new image that has the broom package, you could add a line in your Dockerfile that says RUN R -e "install.packages('broom')" which would make the broom package available in your new Docker image.

ENTRYPOINT is the command to run when starting the image. trestletech/plumber specifies an entrypoint that starts R, plumb()s a file, then run()s the router. If you want to change how plumber starts, or run some extra commands (like add a global processor) before you run the router, you’ll need to provide a custom ENTRYPOINT.

CMD these are the default arguments to provide to ENTRYPOINT. trestletech/plumber uses only the first argument as the name of the file that you want to plumb().

This Dockerfile would just extend the trestletech/plumber image in two ways. First, it RUNs one additional command to install the broom package. Second, it customizes the default CMD argument that will be used when running the image. In this case, you would be expected to mount a Plumber application into the container at /app/plumber.R

You could then build your custom Docker image from this Dockerfile using the command docker build -t myCustomDocker . (where . – the current directory – is the directory where that Dockerfile is stored).

Then you’d be able to use docker run --rm -vpwd:/app myCustomDocker to run your custom image, passing in your application’s directory as a volume mounted at /app.

7.3.3 Automatically Run on Restart

If you want your container to start automatically when your machine is booted, you can use the -d switch for docker run.

docker run -p 1234:8000 -d myCustomDocker would run the custom image you created above automatically every time your machine boots and expose the plumber service on port 1234 of your host machine. Like all other hosting options, you’ll need to make sure that your firewall allows connections on port 1234 if you want others to be able to access your service.

7.4 Docker (Advanced)

If you already have a basic Docker instance running, you may be interested in more advanced configurations capable of hosting multiple plumber applications on a single server and even load-balancing across multiple plumber processes.

In order to coordinate and run multiple Plumber processes on one server, you should install docker-compose on your system. This is not included with some installations of Docker, so you will need to follow these instructions if you are not currently able to run docker-compose on the command-line. Docker Compose helps orchestrate multiple Docker containers. If you’re planning to run more than one Plumber process, you’ll want to use Docker Compose to keep them all alive and route traffic between them.

7.4.1 Multiple Plumber Applications

We’ll use Docker Compose to help us organize multiple Plumber processes. We won’t go into detail about how to use Docker Compose, so if you’re new you should familiarize yourself using the official docs.

You should define a Docker Compose configuration that defines the behavior of every Plumber application that you want to run. You’ll first want to setup a Dockerfile that defines the desired behavior for each of your applications (as we outlined previously. You could use a docker-compose.yml configuration like the following:

More detail on what each of these options does and what other options exist can be found here. This configuration defines two Docker containers that should run app1 and app2. The associated files in this case are layed out on disk as follows:

You can see that app2 is the simpler of the two apps; it just has the plumber definition that should be run through plumb(). So we merely use the default plumber Docker image as its image, and then customize the command to specify where the Plumber API definition can be found in the container. Since we’re mapping our host’s ./app2 to /app inside of the container, the definition would be found in /app/plumber.R. We specify that it should always restart if anything ever happens to the container, and we export port 8000 from the container to port 7001 on the host.

app1 is our more complicated app. It has some extra data in another directory that needs to be loaded, and it has a custom Dockerfile. This could be because it has additional R packages or system dependencies that it requires.

If you now run docker-compose up, Docker Compose will build the referenced images in your config file and then run them. You’ll find that app1 is available on port 7000 of the machine running Docker Compose, and app2 is available on port 7001. If you want these APIs to run in the background and survive restarts of your server, you can use the -d switch just like with docker run.

7.4.2 Multiple Applications on One Port

It may desirable to run all of your Plumber services on a standard port like 80 (for HTTP) or 443 (for HTTPS). In that case, you’d prefer to have a router running on port 80 that can send traffic to the appropriate Plumber API by distinguishing based on a path prefix. Requests for myserver.com/app1/ could be sent to the app1 container, and myserver.org/app2/ could target the app2 container, but both paths would be available on port 80 on your server.

In order to do this, we can use another Docker container running nginx which is configured to route traffic between the two Plumber containers. We’d add the following entry to our docker-compose.yml below the app containers we already have defined.

This uses the nginx Docker image that will be downloaded for you. In order to run nginx in a meaningful way, we have to provide a configuration file and place it in /etc/nginx/nginx.conf, which we do by mounting a local file at that location on the container.

You should set the server_name parameter above to be whatever the public address is of your server. You can save this file as nginx.conf in the same directory as your Compose config file.

Docker Compose is intelligent enough to know to route traffic for http://app1:8000/ to the app1 container, port 8000, so we can leverage that in our config file. Docker containers are able to contact each other on their non-public ports, so we can go directly to port 8000 for both containers. This proxy configuration will trim the prefix off of the request before it sends it on to the applications, so your applications don’t need to know anything about being hosted publicly at a URL that includes the /app1/ or /app2/ prefixes.

We should also get rid of the previous port mappings to ports 7000 and 7001 on our other applications, as we don’t want to expose our APIs on those ports anymore.

If you now run docker compose up again, you’ll see your two application servers running but now have a new nginx server running, as well. And you’ll find that if you visit your server on port 80, you’ll see the “welcome to Nginx!” page. If you access /app1 you’ll be sent to app1 just like we had hoped.

7.4.3 Load Balancing

If you’re expecting a lot of traffic on one application or have an API that’s particularly computationally complex, you may want to distribute the load across multiple R processes running the same Plumber application. Thankfully, we can use Docker Compose for this, as well.

First, we’ll want to create multiple instances of the same application. This is easily accomplished with the docker-compose scale command. You simply run docker-compose scale app1=3 to run three instances of app1. Now we just need to load balance traffic across these three instances.

You could setup the nginx configuration that we already have to balance traffic across this pool of workers, but you would need to manually re-configure and update your nginx instance every time that you need to scale the number up or down, which might be a hassle. Luckily, there’s a more elegant solution.

We can use the dockercloud/haproxy Docker image to automatically balance HTTP traffic across a pool of workers. This image is intelligent enough to listen for workers in your pool arriving or leaving and will automatically remove/add these containers into their pool. Let’s add a new container into our configuration that defines this load balancer

We could export port 80 of our new load balancer to port 80 of our host machine if we solely wanted to load-balance a single application. Alternatively, we can actually use both nginx (to handle the routing of various applications) and HAProxy (to handle the load balancing of a particular application). To do that, we’d merely add a new location block to our nginx.conf file that knows how to send traffic to HAproxy, or modify the existing location block to send traffic to the load balancer instead of going directly to the application.

Where lb is the name of the HAProxy load balancer that we defined in our Compose configuration.

The next time you start/redeploy your Docker Compose cluster, you’ll be balancing your incoming requests to /app1/ across a pool of 1 or more R processes based on whatever you’ve set the scale to be for that application.

Do keep in mind that when using load-balancing that it’s not longer guaranteed that subsequent requests for a particular application will land on the same process. This means that if you maintain any state in your Plumber application (like a global counter, or a user’s session state), you can’t expect that to be shared across the processes that the user might encounter. There are at least three possible solutions to this problem:

Use a more robust means of maintaing state. You could put the state in a database, for instance, that lives outside of your R processes and your Plumber processes could get and save their state externally.

You could serialize the state to the user using (encrypted) session cookies, assuming it’s small enough. In this scenario, your workers would write data back to the user in the form of a cookie, then the user would include that same cookie in its subsequent requests. This works best if the state is going to be set rarely and read often (for instance, the cookie could be set when the user logs in, then read on each request to detect the identity of this user).

You can enable “sticky sessions” in the HAProxy load balancer. This would ensure that each user’s traffic always gets routed to the same worker. The downside of this approach is that it will distribute traffic less evenly. You could end up in a situation in which you have 2 R processes for an application but 90% of your traffic is hitting one of them if it happens the users triggering the majority of the requests are all “stuck” to one particular worker.

7.5 pm2

If you don’t have the luxury of running your Plumber instance on a designated server (as is discussed in the DigitalOcean section) and you’re not comfortable hosting the API in Docker, then you’ll need to find a way to run and manage your Plumber APIs on your server directly.

There are a variety of tools that were built to help manage web hosting in a single-threaded environment like R. Some of the most compelling tools were developed around Ruby (like Phusion Passenger) or Node.js (like Node Supervisor, forever or pm2). Thankfully, many of these tools can be adapted to support managing an R process running a Plumber API.

pm2 is a process manager initially targeting Node.js. Here we’ll show the commands needed to do this in Ubuntu 14.04, but you can use any Operating System or distribution that is supported by pm2. At the end, you’ll have a server that automatically starts your plumber services when booted, restarts them if they ever crash, and even centralizes the logs for your plumber services.

7.5.1 Server Deployment and Preparation

The first thing you’ll need to do, regardless of which process manager you choose, is to deploy the R files containing your plumber applications to the server where they’ll be hosted. Keep in mind that you’ll also need to include any supplemental R files that are source()d in your plumber file, and any other datasets or dependencies that your files have.

You’ll also need to make sure that the R packages you need (and the appropriate versions) are available on the remote server. You can either do this manually by installing those packages or you can consider using a tool like Packrat to help with this.

There are a myriad of features in pm2 that we won’t cover here. It is a good idea to spend some time reading through their documentation to see which features might be of interest to you and to ensure that you understand all the implications of how pm2 hosts services (which user you want to run your processes as, etc.). Their quick-start guide may be especially relevant. For the sake of simplicity, we will do a basic installation here without customizing many of those options.

7.5.2 Install pm2

Now you’re ready to install pm2. pm2 is a package that’s maintained in npm (Node.js’s package management system); it also requires Node.js in order to run. So to start you’ll want to install Node.js. On Ubuntu 14.04, the necessary commands are:

sudo apt-get update
sudo apt-get install nodejs npm

Once you have npm and Node.js installed, you’re ready to install pm2.

sudo npm install -g pm2

This will install pm2 globally (-g) on your server, meaning you should now be able to run pm2 --version and get the version number of pm2 that you’ve installed.

In order to get pm2 to startup your services on boot, you should run sudo pm2 startup which will create the necessary files for your system to run pm2 when you boot your machine.

7.5.3 Wrap Your Plumber File

Once you’ve deployed your Plumber files onto the server, you’ll still need to tell the server how to run your server. You’re probably used to running commands like

pr <-plumb("myfile.R")
pr$run(port=4500)

Unfortunately, pm2 doesn’t understand R scripts natively; however, it is possible to specify a custom interpreter. We can use this feature to launch an R-based wrapper for our plumber file using the Rscript scripting front-end that comes with R. The following script will run the two commands listed above.

Save this R script on your server as something like run-myfile.R. You should also make it executable by changing the permissions on the file using a command like chmod 755 run-myfile.R. You should now execute that file to make sure that it runs the service like you expect. You should be able to make requests to your server on the appropriate port and have the plumber service respond. You can kill the process using Ctrl-c when you’re convinced that it’s working. Make sure the shell script is in a permanent location so that it won’t be erased or modified accidentally. You can consider creating a designated directory for all your plumber services in some directory like /usr/local/plumber, then put all services and their associated Rscript-runners in their own subdirectory like /usr/local/plumber/myfile/.

7.5.4 Introduce Our Service to pm2

We’ll now need to teach pm2 about our Plumber API so that we can put it to work. You can register and configure any number of services with pm2; let’s start with our myfile Plumber service.

You can use the pm2 list command to see which services pm2 is already running. If you run this command now, you’ll see that pm2 doesn’t have any services that it’s in charge of. Once you have the scripts and code stored in the directory where you want them, use the following command to tell pm2 about your service.

You should see some output about pm2 starting an instance of your service, followed by some status information from pm2. If everything worked properly, you’ll see that your new service has been registered and is running. You can see this same output by executing pm2 list again.

Once you’re happy with the pm2 services you have defined, you can use pm2 save to tell pm2 to retain the set of services you have running next time you boot the machine. All of the services you have defined will be automatically restarted for you.

At this point, you have a persistent pm2 service created for your Plumber application. This means that you can reboot your server, or find and kill the underlying R process that your plumber application is using and pm2 will automatically bring a new process in to replace it. This should help guarantee that you always have a Plumber process running on the port number you specified in the shell script. It is a good idea to reboot the server to ensure that everything comes back the way you expected.

You can repeat this process with all the plumber applications you want to deploy, as long as you give each a unique port to run on. Remember that you can’t have more than one service running on a single port. And be sure to pm2 save every time you add services that you want to survive a restart.

7.5.5 Logs and Management

Now that you have your applications defined in pm2, you may want to drill down into them to manage or debug them. If you want to see more information, use the pm2 show command and specify the name of the application from pm2 list. This is usually the same as the name of the shell script you specified, so it may be something like pm2 show run-myfile.

You can peruse this information but keep an eye on the restarts count for your applications. If your application has had to restart many times, that implies that the process is crashing often, which is a sign that there’s a problem in your code.

Thankfully, pm2 automatically manages the log files from your underlying processes. If you ever need to check the log files of a service, you can just run pm2 logs run-myfile, where myfile is again the name of the service obtained from pm2 list. This command will show you the last few lines logged from your process, and then begin streaming any incoming log lines until you exit (Ctrl-c).

If you want a big-picture view of the health of your server and all the pm2 services, you can run pm2 monit which will show you a dashboard of the RAM and CPU usage of all your services.